Cache Fusion Scenarios (RAC).

Block Resource Modes.


Resource Mode  Identifier  Description
Null           N           Nodes which are holding the blocks
                           at this level do not have any access
                           rights to the block.

Shared         S           This level indicates that the block
                           at this level is in shared mode or
                           read mode, Multiple instances have
                           read access to this block but cannot
                           modify it.

Exclusive      X           In this level the block is held in Exclusive
                           mode and other instances cannot write to it.


Block Resource Roles.
 
Role    Description

Local   When the block, is read into the instance's
        cache and no other instance in the cluster
        has read the same block or has it in its cache
        then the block has a local role.
 
Global  If the block which earlier had local role has been modified,
        and because of request from other instances been copied to
        other nodes now has a global role. 
                 

Past Image(PI).

Past Image is a copy of a globally dirty block image maintained in the cache.
it is created when a modified block is served to another instance after
setting the resource role to global.

Current Image (XI).

The current image is a copy of a block held by the current instance
after many modifications and transfers from other instances.


Global Resource Directory(GRD).

The GRD contains information about the current status of all shared resources.

It is maintained by Global Cache Service(GCS) and Global Enqueue Service(GES).

The GRD resides in memory and is distributed throughout the cluster.

Resource Master.

On a RAC cluster, there is only one node that maintains extensive information about
a particular resource (example: block). That particular node is called "Node Master".

The Node Master contains the current information about the precise state of that block.

So, when other nodes request the block, the resource master tells the node
owning the block at that time, to transfer the block to the node requesting it.



Cache Fusion block Transfer Scenarios.
http://www.datadisk.co.uk/html_docs/rac/cache_fusion.htm